1. Field of the Invention
The present invention relates to image projection systems and, more specifically, to correcting projected images.
2. Background Information
Currently, there are a wide-variety of digital image projectors commercially available. Most digital projectors include a video decoder and a light engine. The video decoder converts video data received by the projector, e.g., from the display connection of a personal computer (PC), into pixel and color data. The pixel and color data is then supplied to the light engine, which converts that data into the actual projected image. The light engine includes a lamp, optics and logic for manipulating the light in order to generate the pixels and color.
There are three different types of technologies utilized by the light engines of today""s projectors: Liquid Crystal Display (LCD), Digital Light Processing (DLP) and Liquid Crystal on Silicon (LCOS). An LCD light engine breaks down the light from a lamp into red, green and blue components. Each color is then polarized and sent to one or more liquid crystal panels that turn the pixels on and off, depending on the image being produced. An optic system then recombines the three color signals and projects the final image to a screen or other surface.
DLP technology was developed by Texas Instruments, Inc. of Dallas, Tex. A DLP light engine directs white light from a lamp onto a color wheel producing red, green, blue and white light. The colored light is then passed to a Digital Micromirror Device (DMD), which is an array of miniature mirrors capable of tilting back-and-forth on a hinge. Each mirror corresponds to a pixel of the projected image. To turn a pixel on, the respective mirror reflects the light into the engine""s optics. To turn a pixel off, the mirror reflects the light away from the optics.
A LCOS light engine combines LCD panels with a low cost silicon backplane to obtain resolutions that are typically higher than LCD or DLP projectors. The LCOS light engine has a lamp whose light is sent to a prism, polarized, and then sent to a LCOS chip. The LCOS chip reflects the light into the engine""s optics where the color signals are recombined to form the projected image.
The quality of a projected image is a function of several characteristics, including brightness, also referred to as luminance. Due to the design of the optics within the light engines and/or the lamps themselves, most projectors do not project at a constant luminance level across the entire screen. FIG. 1 is a highly schematic illustration of the luminance levels measured in foot-lamberts of a displayed image 100 that was generated by a projector set to display all pixels at a constant luminance or brightness level. The displayed image 100 has a generally rectangular shape comprising a top edge 102, a right side edge 104, a bottom edge 106 and a left side edge 108. Rather than having a constant luminance throughout, the image 100 has a brightest region 110, which is at approximately 34.0 foot-lamberts, and several regions of decreasing luminance, terminating at a darkest region 112 at the image""s upper left corner, which is at approximately 17.8 foot-lamberts. This non-uniformity in luminance detracts from the displayed image.
The luminance non-uniformity of a projector can become more pronounced under certain conditions. For example, when a xe2x80x9ccompositexe2x80x9d image is created by multiple projectors whose individual images are tiled together, e.g., in a 4 by 5 pattern, to form the composite image, the non-uniformity in luminance is often much more apparent. Luminance non-uniformities can also be created (or existing non-uniformities made more pronounced) when the projector is set up at an angle to the display screen or surface. That portion of the displayed image that is closer to the projector will typically be brighter, while those portions located further away will be dimmer.
Accordingly, a need exists for a projector whose displayed image(s) is uniform in luminosity.
Briefly, the present invention provides a system and method for correcting the luminance non-uniformity of an image generated by a projector. The projector receives input data and projects images based on the input data To correct luminance non-uniformity, a projector correction look-up table (LUT) is created. The LUT attenuates the displayed image such that the whole image has the same luminance as the dimmest point. Input data received by the projector is modified by the correction information contained in the LUT, and the xe2x80x9ccorrectedxe2x80x9d data is used to drive the projector""s light engine, thereby correcting the non-uniformity in luminance.
A camera is preferably used to record the luminance non-uniformity of the projector at all of output levels supported by the projector. For example, if the projector supports 256 output levels, e.g., 0-255, then 256 images are produced by the projector is and captured by the camera. In the preferred embodiment, an attenuation array is computed for the camera in advance to correct for non-uniformities in the camera""s sensors. The images captured by camera, and corrected by the camera""s attenuation array, are organized logically into a three-dimensional (3-D) array of camera capture planes where each plane corresponds to the image captured for a respective output level of the projector. Each camera capture plane is examined, and the location, e.g., in x, y coordinates, of the dimmest captured value is identified. This same location in the projector correction LUT is then assigned the input level of the subject camera capture plane. The values of the remaining locations of the LUT are found by searching down the respective columns of the 3-D camera capture array for the plane whose level matches the dimmest captured value for the plane being evaluated. A suitable interpolation scheme may be used to enlarge the LUT from the resolution of the camera to the resolution of the projector. To improve performance, the LUT may be converted into a one-dimensional (1-D) gain-table, that specifies a gain for each projector output level, and a two-dimensional (2-D) spatial attenuation array.